We are using DNA polymerases obtained by recombinant DNA technology to examine the mechanisms and protein-DNA interactions that are important for determining the fidelity of DNA synthesis. We have determined the fidelity of DNA synthesis catalyzed by the normal Klenow polymerase, by two mutant derivatives lacking proofreading exonuclease activity but having a normal protein structure, and by a protein that contains only one of two domains, the large polymerase domain. The fidelity results have permitted the formulation of four models to explain the production of base substitution and frameshift errors. We have been testing each of these. We have also established the fidelity of the thermostable Taq polymerase used in polymerase chain reactions (PCR), using various reaction conditions, including changes in temperature, pH, relative and absolute dNTP con- centration and Magnesium Chloride, concentration. These studies define high fidelity conditions that are useful for genetic applications of DNA amplified by PCR. We are currently examining mutant derivatives of three recombinant DNA polymerases for which structural information is available, (E. coli DNA polymerase I, HIV-1 reverse transcriptase DNA polymerase beta.) It is our belief that extensive analyses of these enzymes offers the best hope for understanding the molecular determinants for accurate DNA synthesis as well as how this process can be compromised by DNA adducts.